Mathias Mohl has a new video tutorial on AETuts, Creating a Tracked Head Wound:
"In this tutorial Mathias shows that corner pin tracking can do much more than just replacing some pictures on a wall. We are going to compose a wound on a moving head and use the Mocha tracker in combination with his script MochaImport. Trust me, this is so much easier then creating a real head wound!"
Creating a wound with make-up might be easier for some -- see Indy Mogul for techniques. Mathis' scripts MochaImport, KeyTweak, and Tracker2Mask were mentioned previously in AEP posts Mocha Tutorials by Mathias Möhl and AE scripts MochaImport & Final Cut 2 After Effects.